Oral HIV exposure appears to be less risky than intrarectal and intravaginal exposure. Others have shown that saliva inactivates HIV in vitro. We postulate that the oral mucosa is less permeable to HIV than other mucosae due to protective salivary factors. However, mucosal trauma during sexual activity could allow direct HIV access to underlying lymphoid tissues or blood. As trauma could affect different mucosal transmission routes differently, it is difficult to assess the biological role of natural protective factors in the oral cavity in human studies. We propose to challenge anaesthetized rhesus monkeys in a non-traumatic manner with simian-human immunodeficiency virus (SHIV) strains encoding HIV env. We have shown that monkeys of different ages are susceptible to oral SHIV challenge, and X4, dual-tropic X4R5, or R5 strains were infectious orally. R5 SHIVs are the most bioloqically relevant, given that mucosal HIV transmission in humans involves predominantly R5 strains, even when infected source persons harbor mostly X4 or X4R5 strains in the blood. This restriction could be due to 1) exclusive presence of R5 strains in mucosal compartments from where the virus originates; 2) exclusive permeability of mucosal membranes to R5 strains; and/or 3) exclusive spread of R5 strains in the new host after mucosal virus passage. We propose to compare the role of the oral mucosa with that of vaginal and rectal mucosae in transmittinq R5 SHIV in nafve monkeys and in sheddinq virus durinq chronic infection. We have generated valuable reagents and resources for this project: 1) R5 SHIV-A2 that encodes env of a pediatric HIV clade A strain isolated from a Kenyan infant infected orally through breastfeeding; 2) R5 SHIV-1157ip that encodes envof a HIV ctade C strain from a Zambian infant; 3) 13 rhesus monkeys that were orally challenged with SHIV-1157ip and that are currently in the chronic stable phase of infection; and 4) separate funds to titrate SHIV-A2 orally in neonatal monkeys. The Specific Aims are to address the followinq questions: 1. Do mucosal fluids of rhesus monkeys inactivate different SHIVs in vitro and in vivo? Does inactivation correlate with levels of secretory leukocyte protease inhibitor (SLPI)? Is rhesus monkey SLPI similar to human SLPI? 2. What is the relative permeability of intact oral, vaginal, and rectal mucosa to R5 SHIV in adult macaques? Do neonatal monkeys differ in their susceptibility to oral R5 SHIV from that of adults (this question generates no extra costs)? 3. Is R5 SHIV preferentially transmitted orally when the inoculum contains a 1:1 mixture of X4 and R5 SHIV strains? 4. Is R5 SHIV preferentially excreted into milk when lactating macaques are given a 1:1 mixture of R5 and X4 SHIVs? 5. Do mucosal fluids of monkeys dually infected with R5- and X4 SHV strains harbor predominantly the R5 strain? These studies are significant due to their systematic comparison of intact mucosal tissues for their relative susceptibility to a SHIV that encodes env of an orally transmitted African HIV R5 isolate.